1. Field of the Invention
The present invention relates to a sheet post-processing apparatus to apply post-processing to a spine of a folded sheet bundle and an image forming apparatus having the sheet post-processing apparatus. More specifically, the present invention relates to a sheet post-processing apparatus which cosmeticizes a booklet by squaring a spine of a saddle-stitch booklet.
2. Description of the Related Art
Conventionally, when a sheet bundle including about 20 or more sheets as a set is folded at a time, a booklet is formed having a vicinity of a spine being curved obviously. Such a booklet, when folded, easily opens at the side opposite the spine, giving an awkward appearance. Since the booklet cannot be placed flat, it has been difficult to stack many booklets.
To solve this problem, U.S. Pat. No. 6,692,208 proposes a technology of pressing a swelled spine of the booklet and squaring a spine 2a1. The technology of U.S. Pat. No. 6,692,208 will be described with reference to FIGS. 17 and 18. As illustrated in FIG. 17A, conveying units 706 and 707 convey a booklet 2a with its spine 2a1 set to the leading position. The spine 2a1 of the booklet 2a stops against a positioning unit 705 for positioning. Holding portions 702 and 703 nip the booklet 2a. The positioning unit 705 retracts. A pressing roller 704 rotates and moves along the spine 2a1 in a direction A in FIG. 17B while applying a pressure to the spine 2a1 of the booklet 2a. In this manner, the squaring process is applied to the swelled spine 2a1. FIG. 17B is a schematic diagram illustrating the moving direction of the pressing roller 704. The pressing roller 704 retracts to an area free of contact with the booklet 2a until the holding portions 702 and 703 nip the booklet 2a. When the grip portions 702 and 703 nip and hold the booklet 2a, the pressing roller 704 moves from one end to the other end of the booklet 2a as applying pressure to the spine 2a1.
According to the technology described in U.S. Pat. No. 6,692,208, the pressing roller 704 rolls on and presses to deform the spine 2a1 of the booklet 2a held by the holding portions 702 and 703. In FIG. 18, a circumferential velocity V is measured on a pressing surface where the pressing roller 704 is pressed against the spine 2a1 of the booklet 2a. The pressing roller 704 moves along the spine 2a1 at a moving velocity W. In this case, the circumferential velocity V equals the moving velocity W. The pressing roller 704 pushes the spine 2a1 downstream in the moving direction (direction of W in FIG. 18) on the pressing surface between the pressing roller 704 and the spine 2a1 of the booklet 2a. Pressing by the pressing roller 704 deforms and squares the spine 2a1 of the booklet 2a along which the pressing roller 704 has passed. When the pressing roller 704 presses the spine 2a1, the pressing surface of the spine 2a1 of the booklet 2a is pushed in the moving direction (direction of W) of the pressing roller 704 at a supporting point where the holding portions 702 and 703 hold the booklet 2a. As illustrated in FIG. 18, an oblique wrinkle C is generated between the supporting point of the spine 2a1 of the booklet 2a held by the holding portions 702 and 703 and the pressing surface pushed downstream by the pressing roller 704, which causes disfigurement. The wrinkle C is remarkable when the booklet 2a is made of thin paper. When a letter or a picture is printed on the surface near the spine 2a1 of the booklet 2a, an occurrence of the wrinkle C removes the toner applied to the surface of the booklet 2a. The booklet 2a is further disfigured.
The present invention neatly squares a booklet spine without any damage such as a wrinkle on the spine.